We start at the glomerulus – with the composition of the filtrate, the filtration rate, and how the latter is influenced by colloid osmotic pressure. Moving from glomerular to tubular processes, we apply the clearance concept to inulin, urea and other substances and then go on to consider the effects of water reabsorption on the tubular concentrations of inulin and urea. Section 6.6 quantifies the rates of filtration and reabsorption for sodium and bicarbonate in order to bring out some useful generalizations concerning kidney function.

Calculations relating to apparently ‘isosmotic’ reabsorption and the mechanisms and energy requirements of sodium reabsorption (Sections 6.7–6.9) place the emphasis on the bulk movements of water and solutes, rather than on the fine tuning of urine composition. Homeostasis is not neglected, however. Thus, the renal regulation of extracellular fluid volume is approached via autoregulation of glomerular filtration rate (GFR) and glomerulotubular balance (Sections 6.10 and 6.11) and there are calculations on the interrelationships amongst solute excretion, water excretion and urine concentration (Sections 6.12–6.14). In Section 6.15, certain aspects of the medullary countercurrent system are tentatively quantified in an attempt to resolve points that are often left vague in elementary accounts. Finally (Section 6.16), the abundance of mitochondria in the kidney is related to the oxygen cost of sodium reabsorption and to glomerular filtration rate.